This invention relates generally to the field of materials, and more particularly to superalloy materials for use in high temperature applications, and specifically to superalloy materials exhibiting improved weldability.
Nickel and cobalt-based alloys containing chrome, aluminum, titanium and other constituents are well known for use in high temperature applications. Such alloys are commonly referred to as superalloys. The base material of cobalt or nickel is typically present in a concentration of at least 45 wt. % in such materials. Superalloys are commonly used to fabricate gas turbine components that are exposed to very high temperature combustion gases, for example rotating blades and stationary vanes. Superalloys exhibit high strength and corrosion and oxidation resistance at very high temperatures, but they are also known to be susceptible to cracking during fusion welding.
Weldability is an essential material requirement affecting the overall cost of a superalloy component. The initial component fabrication cost is affected by the ease of repairing casting defects and the ability to fabricate multi-piece component assemblies requiring welding. Long term operating cost is affected by the ability to repair a damaged component rather than replacing it. The term weldability is commonly used to describe the ease with which a material may be fusion welded without the formation of cracking. One measure of weldability is the Sigmajig test, which measures the threshold stress for hot cracking. This test was developed at Oak Ridge National Laboratory to quantitatively rank the relative weldability of alloys that are prone to hot cracking. The test involves the application of a transverse stress to a rectangular specimen sheet as an autogenous gas tungsten arc weld is applied across the sheet. Cracking of the specimen will occur when the pre-applied stress exceeds a threshold value. The higher the value of this threshold stress, the more weldable is the material.
Special processes have been developed for improving the weldability of superalloy materials. Such processes generally involve costly pre-heat and/or post weld heat treatments. Many compositions of superalloy materials have been developed in an attempt to achieve good weldability without sacrificing the other beneficial properties of a superalloy material. One such composition is described in U.S. Pat. No. 6,284,392 as a nickel-based alloy containing a specific combination of small amounts of both boron and zirconium.
Scandium has been used in prior art superalloy compositions. U.S. Pat. No. 4,662,920 mentions scandium as one of several elements that may be added to a nickel-based alloy used for handling molten glass in order to provide dispersion strengthening and further corrosion resistance. None of the comparative examples described in that patent actually contain scandium. U.S. Pat. No. 4,261,742 describes a high-chrome, nickel-based, platinum group-containing superalloy including scandium for oxidation/corrosion resistance. Such platinum-containing alloys are generally costly. U.S. Pat. No. 6,007,645 describes a single crystal alloy which may include scandium or other elements for increasing the creep-rupture strength and oxidation and corrosion resistance of the material. Single crystal alloys are known to be difficult to weld, and particularly difficult to weld while retaining a single crystal structure. Nothing in the prior art describes any relationship between scandium and the weldability of a superalloy, nor is there any teaching in the prior art regarding the relationship of scandium and other elements with regard to weldability.
Further improvement in the weldability of superalloy materials is desired. Improved materials should exhibit an increased resistance to hot cracking while maintaining the high temperature strength of known superalloys. Improved materials should avoid the use of the expensive platinum group of elements.
Accordingly, a fusion weldable alloy is described herein as consisting essentially of the composition by weight percent of: chromium 22.0-22.8%; cobalt 18.5-19.5%; titanium 3.6-3.8%; aluminum 1.8-2.0%; tungsten 1.8-2.2%; niobium 0.9-1.1%; tantalum 1.3-1.5%; carbon 0.13-0.17%; zirconium 0.005-0.040%; boron 0.004-0.014%; iron 0.5% maximum; sulfur 0.005% maximum; silver 0.0005% maximum; bismuth 0.00005% maximum; silicon 0.2% maximum; manganese 0.2% maximum; lead 0.005% maximum; nitrogen 0.005% maximum; scandium 0.005-1.0%; and the balance nickel. The nickel-based superalloy may have greater than 0.100% scandium, or greater than 0.300% scandium, or it may have 0.005-0.5% scandium.
Another fusion weldable alloy is described herein as including: at least 45% by weight of at least one of the group of nickel and cobalt; 18-37% by weight chromium; at least 0.005% by weight scandium; and less than 0.04% by weight zirconium. The fusion weldable alloy may further include 0.005-0.040% by weight zirconium. The fusion weldable alloy may include least 0.100% by weight scandium, or at least 0.300% by weight scandium, or it may include 0.005-0.5% by weight scandium, or 0.005-1.0% by weight scandium. The fusion weldable alloy may include less than 3% by weight of the platinum group metals platinum, palladium, rhodium, iridium, osmium and ruthenium. It may further include between 0.001-0.005% by weight boron, and the combination of zirconium, boron and scandium may be in the range of 0.005-0.06%. The fusion weldable alloy may have an MCrAlY bond coat disposed over at least a portion of its surface.
A turbine component is described herein as being made from one of the alloys described above. The turbine component may include an MCrAlY bond coat disposed over a surface of the alloy. The turbine component may further include a fusion weld repaired area formed in the alloy.